Containers made of iron used mainly in the field of beverage cans may be classified as two-piece cans and three-piece cans.
Two-piece cans are can bodies in which the can bottom and the can wall have been formed as a single piece, and are represented by DrD (draw and redraw) cans, DI (drawing and ironing) cans, etc. These cans may be formed by drawing, ironing, bending and reverse bending, or a combination thereof. Steel sheets to be used for these can bodies may include tin plates (Sn-plated steel sheets) and TFS (electrolytic chromate-treated steel sheets (tin-free steel)), and these steel sheets may be used depending on the applications and processing methods used therefor.
Three-piece cans are can bodies in which the can wall and the can bottom thereof have been formed as separate pieces. Three-piece cans may be mainly in the form of welded cans in which the can wall is formed by welding. As the material for the can wall, lightly coated Sn-plated steel sheets and Ni-plated steel sheets may be employed. As the material for the can bottom, TFS, etc, may be employed.
In both the two-piece can and the three-piece can, the outside surface of the can is provided with printing, in order to appeal to consumers for commercial value of the canned goods. On the other hand, the inside surface of the can is coated with a resin so as to ensure the corrosion resistance of the can body. In the case of the two-piece can in the prior art, after the formation of the can body, the inside surface of the can is coated, for example, by spraying and the outside surface of the can is subjected to curved surface printing. Recently, it is common to use laminated two-piece cans in which the can is formed from a steel sheet which has preliminarily been laminated with a PET film (Patent Document 1 and Patent Document 2).
In addition, with respect to the welded cans for constituting the three-piece cans, the can body is hitherto produced by welding steel sheets, in which the outside surface of the can, as well as the inside surface of the can, has preliminarily been printed. However, instead of the painting or painting finish, it is common to use three-piece cans which are produced by using steel sheets (i.e., laminated steel sheets), which have preliminarily been provided with lamination with a printed PET film (Patent Document 3 and Patent Document 4).
In the production of two-piece cans, a steel sheet for a container is subjected to drawing, ironing, or bending and reverse bending. In the production of three-piece cans, a steel sheet for a container is subjected to neck forming or flanging. Further, in some cases, the steel sheet for a container is also subjected to expanding for the purpose of imparting a design to the can. Therefore, the laminated steel sheet used as a steel sheet for a container must have excellent adhesion to a film so that the laminated steel sheet can follow these processes.
Sn-plated steel sheets have excellent corrosion resistance, even with respect to an acidic content, due to the excellent sacrificial anticorrosive effects of the Sn. However, Sn-plated steel sheets do not exhibit a stable adhesion with a film because they have brittle Sn oxides present on their outermost surface layer. As a result, when Sn-plated sheets have been subjected to the above-described processings, there are problems that peeling of the film is caused, corrosion begins at sites where the adhesion strength between the film and the steel sheet is not sufficient.
Thus, a Ni-plated steel sheet which not only has excellent processability and adhesion, but also is capable of being welded is used as a laminated steel sheet for a container (Patent Documents 5). Ni-plated steel sheets have been disclosed for a long time (for example, Patent Documents 9). Some Ni-plated steel sheets have dull surfaces as in the case of Sn-plated steel sheets, while there are also ones which have been subjected to bright plating by Ni plating methods in which a brightening agent is added (Patent Document 6 and Patent Document 7).
However, since Ni does not exhibit any sacrificial anticorrosive effect such as Sn, it is known that in the case of Ni-plated steel sheets, highly corrosive contents, such as acidic drinks, cause pitting corrosion (or perforation corrosion), in which the corrosion grows in the sheet depth direction due to defects in the Ni plating layer, such as pinholes, leading to perforation. Therefore, there has been a need to improve the corrosion resistance of Ni-plated steel sheets. In order to reduce pitting corrosion, a Ni-plated steel sheet was developed in which the steel components were adjusted so that the electric potential of a steel sheet to be plated was more noble (Patent Document 8).